This invention pertains to systems that monitor control signals. In complicated machinery, it is common practice to have a collection of switches which open and close depending upon the state of the machine. For example, in an eccentric press, the use of a dual-plunger electrically operated solenoid valve is established. Such a valve is so arranged that when the valve is operating properly, the plungers will move in and out together. It is possible to provide two switches, with one switch being associated with each plunger and being actuated thereby to turn on and off depending upon the plunger's position. Such a valve is conventionally energized by a main switch, the status of which can also be ascertained by an additional third switch. Thus, it is possible to ascertain the actual status of the valve and the main switch by use of control signals which are turned on and off by auxiliary switches which are associated with the valve and the main switch.
In the event that it is desired to supervise such control signals, it is of great importance that the system which performs the supervision inform an operator when the system itself malfunctions. Otherwise, it is possible that the system through its own malfunction would cease to serve as a supervisor of the actual status of the main switch and the control valve and would therefore permit the machine to operate even though a malfunction had occurred.